1. Field of the Invention
The present invention relates to a registration accuracy measurement mark, and particularly to a registration accuracy measurement mark used for measuring overlay accuracy in the lithography process in the manufacturing process of semiconductor devices.
2. Description of the Background Art
According to a conventional method for measuring the overlay accuracy relative to two lithography steps in a manufacturing process of a semiconductor device, a pattern of a registration accuracy measurement mark is formed simultaneously with a master pattern (referring to a pattern to be actually produced) using the same mask in each lithography step, in order to measure the overlay accuracy of the master pattern by the overlay accuracy of the registration accuracy measurement mark pattern. A conventional registration accuracy measurement mark is hereinafter described referring to FIGS. 10-15.
Referring to FIG. 10, the conventional registration accuracy measurement mark is arranged in a scribe line region 112 adjacent to a circuit region 111 where a master pattern 101 is to be formed. Referring to FIG. 11, a master pattern 105a of the first layer and a registration accuracy measurement mark pattern 105b of the first layer are partitioned by a lowered region generated by a groove 110 of scribe line region 112. Master pattern 101 of the second layer is placed on the first layer master pattern 105a and a registration accuracy measurement mark pattern 102 of the second layer is placed on the first layer registration accuracy measurement mark pattern 105b as a line located along the four sides of a rectangle to have its part parallel to the second layer master pattern 101.
In the case of such a registration accuracy measurement mark, since the width of the second layer registration accuracy measurement mark pattern 102 is determined independently of the width of the second layer master pattern 101, the amount of offset (amount of displacement) due to aberration, (i.e., the difference between the position of an optical from by a lens and ideal optical image) is different between the master pattern and the registration accuracy measurement mark pattern both at the second layer when the patterns are generated. A phenomenon thus observed upon generation of the second layer registration accuracy measurement mark pattern 102 and master pattern 101 is that the relative positions of those patterns at the second layer are different from those of the respective patterns on a mask pattern. This phenomenon is described below.
The light passing through a mask usually advances with a prescribed diffraction angle according to the size of each opening of the mask, after transmitting through the opening. Each pattern thus generated by the mask is displaced by the diffraction angle of the transmitted light, resulting in a phenomenon where offset due to the aberration is observed.
The diffraction angle of the light transmitted through the opening formed as a wider line on the mask is different from that of the light transmitted through the opening formed as a narrower line. Therefore, if the width of the master pattern is different from that of the registration accuracy measurement mark pattern, the amount of offset attributable to the aberration would be different. The difference in the amount of offset due to aberration causes the difference between the master pattern and the registration accuracy measurement mark pattern in the amount of offset in registration (alignment). The difference in the amount of offset due to aberration thus influences registration.
In order to avoid this influence of the difference in the amount of offset due to aberration, a master pattern formed as lines and a registration accuracy measurement mark pattern formed as lines are produced to have the same width as shown in FIGS. 12 and 13. In this case, the same width of the second layer registration accuracy measurement mark pattern 102 and the second layer master pattern 101 allows the amount of offset due to aberration to be the same, so that the second layer registration accuracy measurement mark pattern 102 and the second layer master pattern 101 are generated to have the relative spacing between their lines which is the same as that of the opening pattern formed on the mask.
In the above-described case shown in FIGS. 12 and 13, it is unnecessary to consider the depth of focus since the height of the surface of the first layer registration accuracy measurement mark pattern 105bon which the second layer registration accuracy measurement mark pattern 102 is formed coincides with that of the first layer master pattern 105a on which the second layer master pattern 101 is formed.
On the contrary, referring to FIGS. 14 and 15, if the first layer registration accuracy measurement mark pattern 105b on which the second layer registration accuracy measurement mark pattern 102 is formed and the first layer master pattern 105a on which the second layer master pattern 101 is formed are produced to have different heights, the depth of focus that compensates for the difference in height cannot be secured. As a result, the second layer registration accuracy measurement mark pattern 102 cannot be produced accurately due to the out of focus state.
According to a design usually employed, the difference in height between the surface of the first layer registration accuracy measurement mark pattern 105b on which the second layer registration accuracy measurement mark pattern 101 is formed and that of the first layer master pattern 105a on which the second layer master pattern 101 is formed is approximately 0.6 xcexcm at maximum in both cases where the first layer registration accuracy measurement mark pattern is higher than the first layer master pattern and vice versa. Accordingly, if the depth of focus of more than approximately 1.2 xcexcm is ensured, the pattern generation is possible in the focused state even if there is a difference in height between the surface of the first layer registration accuracy measurement mark pattern 105b and that of the first layer master pattern 105a. 
In order to increase the depth of focus, the width of the second layer registration accuracy measurement mark pattern 102 is made larger than that of the second layer master pattern 101. According to this method, an opening width of a mask for generating the second layer registration accuracy measurement mark pattern 102 is made larger than that for generating the second layer master pattern 101, so that the diffraction angle of the light transmitted through the opening of the mask for generating the second layer registration accuracy measurement mark pattern 102 is reduced. Consequently, the angle of light incident on the surface of the substrate is closer to the right angle relative to the surface to achieve increase in the depth of focus.
However, if the second layer registration accuracy measurement mark pattern 102 is made too wider than the second layer master pattern 101 for securing the increased depth of focus, the influence of the difference in the amount of displacement due to aberration also increases. Accordingly, the width of the second layer registration accuracy measurement mark pattern 102 should be made larger than that of the second layer master pattern 101 such that it allows measurement in the overlay inspection to be within the range of an allowable measurement error and allows the registration accuracy measurement mark pattern to be formed with sufficient depth of focus ensured, even if there is a formation error of patterns due to the difference in the amount of offset induced by aberration.
The allowable measurement error in the amount of offset in registration is now described. Usually, if the line-like master pattern with its dimension of approximately 0.2 xcexcm to 1.0 xcexcm is used, the numerical standard of the allowable measurement error in the amount of offset in registration is 60 nm. It means that measurement errors should be 60 nm or less in total. Those measurement errors are induced by the measurement error attributable to the exposure system, the measurement error attributable to the wafer, and the formation error of the pattern due to the difference in the amount of offset attributable to aberration of light. In the allowable measurement error of 60 nm, the measurement error due to the offset, rotation and magnification of the exposure system (stepper) is about 25 nm, and the measurement error due to the wafer component is about 30 nm. In view of this, the formation error of the pattern due to difference in the amount of offset attributable to aberration should be 5 nm or less.
One object of the present invention is to provide a semiconductor device having a registration accuracy measurement mark that allows measurement in overlay inspection to be done within an allowable measurement error even if a master pattern and a registration accuracy measurement mark pattern are formed with a difference in level.
The semiconductor device according to the present invention is provided with a registration accuracy measurement mark for measuring an error in registration relative to lithography steps in a lithography process of the semiconductor device. The difference of width in the same direction between the master pattern and the registration accuracy measurement mark pattern is determined such that the difference in the amount of offset between the master pattern and the registration accuracy measurement mark pattern induced by aberration in transfer of the patterns is at most a prescribed value, and such that the width of the registration accuracy measurement mark pattern in the same direction as that of the master pattern is larger than the width of the master pattern by at least a prescribed value to ensure the depth of focus considering the difference in level of the master pattern and the registration accuracy measurement mark pattern on a substrate.
This arrangement described above enables the difference in the amount of offset between the master pattern and the registration accuracy measurement mark pattern due to aberration in transfer of the patterns to be a prescribed value or less. Further, a sufficient depth of focus considering the difference in height on the substrate between the master pattern and the registration accuracy measurement mark pattern can be secured. Accordingly, even if the master pattern and the registration accuracy measurement mark pattern are formed with a level difference therebetween, the registration accuracy measurement mark is produced with the sufficient depth of focus ensured, achieving an accurate formation of the registration accuracy measurement mark. In addition, since the registration accuracy measurement mark can be produced such that the difference in the amount of offset due to aberration falls within the allowable formation error, measurement in the overlay inspection can be done within the range of the allowable measurement error.
The semiconductor device provided with the registration accuracy measurement mark according to the present invention may have a master pattern formed as a line with its width of 0.2 xcexcm to 1.0 xcexcm and have a registration accuracy measurement mark pattern formed as a line with its width larger than that of the line-like master pattern by 0.085 xcexcm to 0.1 xcexcm.
The amount of offset due to aberration usually has a fixed value for each line size. If the width of the line-like registration accuracy measurement mark pattern and that of the line-like master pattern are determined, the difference in the amount of offset due to aberration between respective patterns is determined. By setting the difference between the width of the line-like registration accuracy measurement mark pattern and that of the master pattern to be 0.1 xcexcm or less as described above, the difference in the amount of offset due to aberration can be at most the allowable formation error (5 nm), with the width of the line-like master pattern of 0.2 xcexcm to 1.0 xcexcm considering the relation between the line size and the amount of offset due to aberration.
The depth of focus also has a fixed value for each line size. As the line size becomes larger, the depth of focus increases. By setting the width of the line-like registration accuracy measurement mark pattern to be larger than that of the line-like master pattern by 0.085 xcexcm or more, the sufficient depth of focus (1.2 xcexcm) can be ensured even if there is a difference in height (maximum 0.6 xcexcm) between the surfaces of the master pattern and registration accuracy measurement mark pattern, considering the relation between the line size and the depth of focus.
The semiconductor device provided with the registration accuracy measurement mark according to the present invention may have a master pattern formed as a hole with its opening width of 0.3 xcexcm to 1.0 xcexcm and have a pattern of the registration accuracy measurement mark with its opening width larger than that of the hole-like master pattern by 0.1 xcexcm or less.
The amount of offset due to aberration usually has a fixed value for each hole size. If the opening widths of the hole-like registration accuracy measurement mark pattern and the hole-like master pattern are determined, the difference in the amount of offset due to aberration of respective patterns is determined. By setting the difference between the opening width of the hole-like registration accuracy measurement mark pattern and that of the hole-like master pattern to be 0.1 xcexcm or less, the difference in the amount of offset due to aberration can be made equal to or less than the allowable formation error (5 nm) with the opening width of the hole-like master pattern of 0.3 xcexcm to 1.0 xcexcm considering the relation between the hole size and the amount of offset due to aberration.
The depth of focus also has a fixed value for each hole size. As the hole size becomes larger, the depth of focus increases. By setting the opening width of the hole-like registration accuracy measurement mark pattern to be larger than that of the hole-like master pattern as described above, the sufficient depth of focus (1.2 xcexcm) can be secured even if there is a difference in height (maximum 0.6 xcexcm) between the surface of the hole-like master pattern and that of the registration accuracy measurement mark pattern, considering the relation between the hole size and the depth of focus.
The opening width refers to the diameter of a circle if the hole pattern is formed as a circle, and to the length of one side if the hole pattern is rectangular.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.